1. Field of the Invention
The present invention relates to a time synchronizing system for unifying the time between a plurality of distributed control oriented terminal devices.
2. Related Background Art
Generally, what is dominant among a variety of plant monitor control systems is a distributed control system that includes, with developments of a microcomputer and a signal transmission technology, a plurality of terminal devices connected to each other via transmission lines. Further, in this type of monitor control system, an occurrence time data when a state change is added to various categories of input data given from the plant and recorded, and the state is thus displayed.
When the plant data received from the plant are transmitted to other terminal device via the transmission line from one terminal device, it is required that the time data be added.
In the terminal device receiving first the input data, a time signal is added in a state variation of the input data, time synchronization as precise as a required time resolution or higher needs to be taken between the terminal devices, and the prior art involves the use of transmission lines for connecting the terminal devices or signal lines dedicated to the time synchronization. In this case, the system is configured as a transmission system that accounts for the time synchronization, or hardware and software dedicated to the time synchronization are needed. This might induce a decrease in data transmission efficiency, and a decline of economical value, and might lose a continuity of time because of the time itself being rewritten when correcting the time.
Thus, according to the conventional method of adding the time signal to the input data, if a time resolution having a accuracy as high as 1 ms is required, a time synchronization signal process as precise as this resolution or higher needs to be executed. Nevertheless, it is difficult to actualize this and ensure the accuracy.